To snow, or not to snow: that is the question (for later this weekend):

For the last several days, various meteorological forecast tools have indicated at least slight potential for an East coast storm in the Sunday to Monday (1/13-14) timeframe. However, most forecast guidance indicated this to be a very low probability event, with "stormy" solutions at times outnumbered 12 to 1 by "benign" or "non-stormy" solutions. If a "stormy" solution were to verify, however, several inches of snowfall could result over parts of the mid-Atlantic region (including Pennsylvania) and New England.

Forecasts from 1 pm Thursday January 10, 2008 continued to indicate considerable uncertainty in their solutions for Sunday-Monday, January 13-14, 2008. To illustrate, take a look at the 84-hour GFS model forecast valid at 06z (1AM EST) Monday, January 14, 2008. This particular solution is perhaps the most aggressive solution of any currently available guidance, showing a deepening area of low pressure off of the New Jersey coast with moderate to heavy snowfall over the eastern third of Pennsylvania and much of southern New England and New Jersey.

On the other hand, now look at the 84-hour NAM model forecast valid at the same time, and note the drastic differences compared to the GFS. The NAM simulates a weaker surface low that is much further offshore, sparing Pennsylvania any heavy precipitation from the ocean storm, while producing only spotty light snowfall from the Ohio Valley through north central Pennsylvania. Fortunately, meteorologists can assess the likelihood and confidence of forecast scenarios by consulting computer ensemble forecasts.

The era of ensemble forecasting allows meteorologists to look at an "envelope" of possible solutions from a particular computer model or cluster of forecast models. To end up with the range of possible solutions, observational data is slightly tweaked before being fed into the model to see how these slight differences might affect the outcomes. Each outcome that results from slightly different initial conditions is referred to as an ensemble member

Now take a look at the 10 January 2008 18z (1PM EST) Medium Range Ensemble Forecast (MREF) plot of individual member precipitation forecasts for Harrisburg valid from 11 January 2008 to 19 January 2008. Blue lines indicate precipitation type in the form of snow, while green indicates rain. Precipitation amounts are labeled along the left axis, while dates are labeled at the bottom. Instantaneous precipitation output from individual ensemble members is shown in gray.

Note that about 7 individual members for Harrisburg are clustered together with higher precipitation amounts (all but one indicating snow), ranging from 0.6" to 0.9" water equivalent, while the remainder of the members are clustered between .25" and .4" of W.E., most indicating snow. What this suggests is increasing confidence in at least a light precipitation event for Harrisburg, most likely in the form of snow, with a couple to several inches of snowfall possible. This plot also indicates a roughly 1 in 6 chance of greater than 6 inches of snowfall for Harrisburg from Sunday night into Monday.

MREF plumes to the west of Harrisburg (not shown) are consistent in producing
at least a light precipitation event Sunday night into Monday, with most
if not all members indicating snow across central and western Pennsylvania
and accumulations up to a couple of inches. There are just a very few outlier
solutions indicating the slight potential for somewhat heavier amounts across
central Pennsylvania, but confidence in this is low given only isolated
members depicting this solution. To the east and south of Harrisburg, similar
distributions of precipitation amounts are seen, with more members indicating
precipitation in the form of rain as you move southeast of Lancaster towards
Philadelphia. Essentially, the plumes are split 50/50 between rain and snow
for far southeast Pennsylvania.

To conclude, as of Thursday evening, January 10, 2008, considerable uncertainty exists regarding the late weekend storm potential. Several of our computer model runs develop a coastal snowstorm during that time frame, but at least 50 percent of these keep the low far enough out to sea to have little effect on Pennsylvania. The most likely scenario is for some light snowfall to affect most of central Pennsylvania associated with a much weaker disturbance tracking across the Ohio Valley during the same timeframe, but there is still some potential for heavier amounts, especially over the eastern third of Pennsylvania, as indicated by the MREF plumes.

We shall see how the situation evolves in the next day or two, but as of
this time, after having read this, you hopefully have a much better sense
of the factors that influence storm forecasts in general, as well as the
potential and uncertainty regarding forecasts for late this weekend and
early next week. Stay tuned.